A typical disk file linear VCM actuator comprises a coil movable through the magnetic field of a permanent magnetic stator. The VCM includes a fixed magnetically permeable center core and a magnetically permeable outer core surrounding and spaced from the center core. Permanent magnets are attached to the outer core and spaced from the center core to define a working gap through which magnetic flux passes. A movable coil is supported within the working gap and is connected to a carriage which supports the read/write heads. The application of current of proper magnitude and polarity to the coil produces a magnetic field which reacts with the magnetic field in the working gap to apply force to the coil and carriage to move the attached read/write heads radially relative to the disks.
In disk files which have a relatively high density of data tracks on the disks, a servo control system is typically used to position the heads to the desired data tracks in the minimum time and to maintain the heads precisely over the center lines of the desired tracks during read or write operations. In order to determine the value of the control current to be applied to the coil of the VCM, it is necessary to know the relationship between the current applied to the VCM and the acceleration of the coil and carriage. This relationship is given by the acceleration factor (K.sub.f /M) which defines the acceleration of the coil and carriage per unit of input current. The term K.sub.f is the force factor which relates force applied to the coil of the input current, and M is the mass of the movable portion of the VCM (i.e. the coil, carriage, and attached head/arm assemblies). The value of the force factor K.sub.f typically varies with the linear stroke position of the coil in the gap of the VCM. In addition, the profile of K.sub.f with position is different for different directions of movement. Conventional servo control systems cannot compensate for any variation in K.sub.f with position, but rather assume a constant K.sub.f regardless of the actual K.sub.f. For this reason, it is desirable to design the VCM in order to make K.sub.f constant with position, as well as to make the K.sub.f profile the same regardless of the direction of movement.
Certain disk file linear VCMs have a dual magnetic flux path through which the coil moves. In such disk files the variation K.sub.f with position and direction is accentuated because at certain times the coil is subjected to the magnetic flux from both magnetic flux paths.